Start by trimming the fat off a beef chuck roast and cut it into 2-inch pieces. Season the beef with salt and pepper.
In a slow cooker, mix together adobo sauce, beef broth, apple cider vinegar, lime juice, half of the onion, garlic, cumin, oregano and bay leaves.
Add the beef to the slow cooker and stir to coat it with the sauce.
Cook the beef on low for 8-10 hours or on high for 4-6 hours, until it is tender and falls apart easily with a fork.
Once the beef is cooked, remove it from the slow cooker and shred it with two forks.
Pour the leftover sauce from the slow cooker over the shredded beef, along with the cilantro.
Toast the buns, if you want. Top them with barbacoa, onion, tomato, and chipotle mayo.